These investigations are devoted to the development of non- invasive methods of accessing cellular structure and function. Two general techniques are being used: Nuclear Magnetic Resonance (NMR) and optical spectroscopy. Over the last year we have developed a new NMR imaging technique which permits the imaging of chemical reaction rates as well as detect the presence of molecules in concentrations as low as 1 umolar due to their chemical exchange with substances in larger concentration. Different methods of determining the flux of creatine kinase in vivo have been evaluated and quantitatively compared. We have also developed a NMR microscopic technique which has permitted the to imaging of small vessels and structures in the in vivo kidney. NMR imaging and flow studies using 2H was developed and demonstrated in the in vivo environment. Using optical spectroscopy, a procedure for monitoring NADH fluorescence of the surface of the heart was developed along with the use of an trapped internal reference compound.